To obtain hydrocarbons, a drilling tool is driven into the ground surface to create a wellbore through which the hydrocarbons are extracted. Typically, a drill string is suspended within the wellbore. The drill string has a drill bit at a lower end of the drill string. The drill string extends from the surface to the drill bit. The drill string has a bottom hole assembly (BHA) located proximate to the drill bit.
Measurements of drilling conditions, such as, for example, an inclination and an azimuth, a drift of the drill bit, fluid flow rates and fluid composition, may be necessary for adjustment of operating parameters, such as, for example, a trajectory of the wellbore, flow rates, wellbore pressures, production rates and the like. The BHA has tools that may generate and/or may obtain the measurements of the drilling conditions. For example, the BHA may acquire information regarding the wellbore and subsurface formations. Technology for transmitting information within a wellbore, known as telemetry technology, is used to transmit the information from the tools of the BHA to the surface for analysis. The information may be used to control the tools. Adjustment of the drilling operations in response to accurate real-time information regarding the tools, the wellbore, the formations and the drilling conditions may enable optimization of the drilling process to increase a rate of penetration of the drill bit, reduce a drilling time and/or optimize a placement of the wellbore.
High angle wells and horizontal wells increase retrieval of the hydrocarbons and improve recovery of the area in which the wellbore is located. To optimize the placement of these wells, the wellbore must be drilled into the target reservoir at the appropriate depth. Typically, information derived from a seismic survey is used to ensure that the wellbore is drilled in the target reservoir at the appropriate depth. In addition, the seismic survey may indicate properties of a region located beneath the drill bit to enable adjustment of the drilling operations, such as, for example, determination of a distance to drill before setting the next string of casing.
The seismic survey may be obtained by processing reflected seismic waves generated by subsurface seismic reflectors, such as, for example, the top and the bottom of the target reservoir. The reflected seismic waves are typically generated by a seismic acoustic source located at the surface, either on land, as generally shown in FIG. 1A, or offshore, as generally shown in FIG. 1B. When a seismic wave encounters a boundary between two materials having different impedances, a portion of the seismic wave is reflected and a portion of the seismic wave is transmitted through the boundary. Processing of the reflected seismic waves provides a graph indicating a surface seismic section of the area as a function of time as generally shown in FIG. 1C. However, the graph merely indicates the time the reflected acoustic waves were acquired and does not indicate a depth at which the reflected acoustic waves were reflected.
Before the well is drilled in the target reservoir, an accurate depth of the location of the subsurface reflectors associated with the target reservoir is unknown. Typically, the two-way travel time of acoustic waves measured at the surface during seismic survey acquisition and processing only enable association of a time with each of the subsurface seismic reflectors in the time domain. For example, a specific reflector may be associated with a time of 5 milliseconds. However, a great deal of uncertainty remains as to the true (or actual) depth of the target reservoir (or any other subsurface layer).